To date, hologram recording systems have been known as a digital information recording system that utilizes the principle of hologram. A feature of this system is to record an information signal into a recording medium as a change in a refractive index. Photorefractive materials such as a single crystal lithium niobate or the like are used for the recording medium. In a hologram recording medium, data can be recorded and reproduced in the units of two-dimensional plane pages, and multiplexed recording is possible by using a plurality of pages. The outline of the recording medium system is explained below.
At the time of recording, in a conventional 4f system hologram recording and reproducing apparatus, a laser light beam 12 emanating from a laser light source 11 is split into lights 12a and 12b by a beam splitter 13, as shown in FIG. 1. The light 12a is shaped into a substantially collimated light, the beam diameter of which is enlarged by a beam expander BX, and is projected onto a spatial light modulator (SLM) such as a transmissive-type TFT liquid crystal display (Thin Film Transistor Liquid Crystal Display) (hereinafter also referred to as “LCD”) panel or the like. An encoder 25 converts a digital data to be recorded in a recording medium 10 into an bright and dark dot-pattern image on a plane and rearranges it into a data array of, for example, 480 vertical bits×640 horizontal bits. The encoder generates a unit-page series data and sends out the data to the spatial light modulator SLM.
When the light 12a transmits through the spatial light modulator SLM, it is light-modulated and turned into a signal light containing a data signal component. The signal light 12a containing the dot pattern signal component passes through a Fourier transform lens 16, which is spaced apart by its focal distance f, and the dot pattern signal component is Fourier transformed. Then, the light is gathered into a recording medium 10.
On the other hand, the light beam 12b split by the beam splitter 13 is guided as a reference light into the recording medium 10 by mirrors 18 and 19, and it intersects the light path of the signal light 12a within the recording medium 10, forming a light interference pattern. Thus the entirety of the light interference pattern is recorded as a change in the refractive index (refractive index grating). In addition, it becomes possible to record a plurality of two-dimensional plane data with angle multiplexing by varying the incident angle of the reference light 12b onto the recording medium 10.
At the time of reproducing, inverse Fourier transform is performed to reproduce the dot pattern image. As shown in FIG. 1, for example, the light path of the signal light 12a is blocked by the spatial light modulator SLM so that only the reference light 12b is projected onto the recording medium 10. The reference light 12b is controlled by the mirror driven in the position and angle thereof with a combination of the rotation and linear movement so that the incident angle thereof results in the same as that of the reference light at the time when the page to be reproduced has been recorded. A reproduced light that reconstructs the recorded light interference pattern appears on a side of the recording medium 10 that is opposite the side thereof that is irradiated with the reference light 12b. When this reproduced light is guided to the inverse Fourier transform lens 16a and is inverse Fourier-transformed, the dot pattern image can be reconstructed. Further, this dot pattern image is received by a photo-detector 20 such as a charge coupled device (CCD) or the like at the focal distance position, and the image is reconverted into an electrical digital data signal. Thereafter, the data signal is sent to a decoder 26, and the original page data is reproduced.
In the recording and reproducing system shown in FIG. 1, according to the rules of Fourier transform and inverse Fourier transform, the transmitted light for, for example, the portion of the image data “A” as shown in FIG. 2(a) that is displayed on the spatial light modulator SLM is Fourier-transformed and recorded into the recording medium as an interference pattern of Fourier transform pattern, and the image of the image data A that has been inverse Fourier-transformed as shown in FIG. 2(b) is reproduced on the CCD 20 from the recording medium illuminated with the reference light. Therefore, the conventional recording and reproducing system necessitates a CCD 20 that is similar to the spatial light modulator SLM with 480 vertical bits×640 horizontal bits and has the same resolution. The precondition is that the recording and reproducing system uses a fixed conversion rule for the recording system and the reproducing system to perform recording and reproducing.
For this reason, it is required for the conventional recording and reproducing system to keep optical distortion, deviation of the signal image, or the like that occurs in the Fourier transform optical system, the inverse Fourier transform optical system, and other optical systems, within a predetermined specified value range. This requires such components as high-precision lenses or the like for the optical systems, and moreover a high-precision relative position adjustment is necessary. Furthermore, since the transfer of pixel data is performed, an expensive detector such as a CCD or the like is required in order to perform high-speed data transfer.
Accordingly, an example of the problem that the present invention intends to solve is to provide a hologram recording and reproducing system that does not require an inverse Fourier lens.